Phase-regulated power and frequency control of an ultrasonic transducer can use a variable frequency oscillator of a phase control circuit with voltage pulses amplified by a driver. In that process first the frequency of the oscillator is varied by a wobbler to find the resonance of the ultrasonic transducer and the scanner is locked to the resonance frequency of the ultrasonic transducer after locking in the phase control circuit.
German Pat. No. 34 01 735 describes an apparatus which has been proven to be effective in practice and particularly eliminates the numerous outstanding problems and difficulties which had earlier existed in operating ultrasonic transducers.
As is known in applications of ultrasonic transducers for liquid atomizers or for welding purposes the oscillator supplying the excitation frequency for the transducer must be able to adjust to numerous different operating properties of the piezoelectric or magnetostrictive transducers.
Changes of the resonance frequency of the transducer can occur which depend on the load on the transducer, on the temperature and on the aging of the piezoceramic and/or the magnetostrictive material.
Further impedance changes of the transducer can be produced by the specific properties of the transducer material, especially the physical properties of the piezodisks.
Finally changes of phase angle between the voltage and current in the transducer may occur which are likewise dependent on the excitation frequency, the load, the amplitude and the temperature. These phenomena occur in practical applications so that the oscillator must be adjusted for the given changes of operating conditions.
In the apparatus described in German Patent 34 01 735 this succeeds because of the use of a phase control circuit. The transient start-up oscillations of the transducer under heavy damping cause difficulties however, when for example residual fluid droplets are found on the transducer or start-up liquid not sprayed flows along the transducer before oscillation.
Then frequently the excitation energy available is not sufficient to permit the start-up of oscillation. A general increase of the start-up oscillation power has in contrast the disadvantage of uneconomical operation and could mask the danger of an overload of the transducer.
Besides the oscillation amplitude influenced by the transducer power also determines the droplet size, which in itself is usually determined by the application, so that on this basis the limits of the free variation of the excitation power are set.
Finally an ultrasonic transducer should be operated with constant oscillation amplitude to maintain a uniform droplet spectrum in liquid atomization.